1. Field of the Invention
The invention relates to an antenna device that radiates or receives radio waves. Specifically, the invention relates to an antenna device that may be applied to a tire pressure detecting device of an automobile.
2. Description of the Related Art
In an existing art, an antenna device described in Japanese Patent Application Publication No. 2001-24431 (JP-A-2001-24431) is known. The antenna device is formed of a feeding antenna disposed upright at the center of a grounded conductor and a plurality of parasitic antennas that are disposed upright around the feeding antenna and that have variably controllable reactances. In this antenna device, the reactances of the surrounding parasitic antennas are electrically varied to thereby control the directivity of the antenna device.
Japanese Patent Application Publication No. 11-88246 (JP-A-11-88246) describes an antenna for a small mobile receiver. In the antenna, loop antennas are provided on different sides of a rectangular parallelepiped box, and then radio waves to be received by the respective loop antennas are switched to receive radio waves to thereby improve the sensitivity of the antenna.
Japanese Patent Application Publication No. 2003-298331 (JP-A-2003-298331) describes an antenna device for a wireless mouse. In the antenna device, loop antennas are arranged respectively on two sides perpendicular to each other, and a directivity of the antenna device is switched to the directivity corresponds to the selected loop antenna to implement resistance to noise.
Japanese Patent Application Publication No. 2005-162192 (JP-A-2005-162192) describes a tire pressure detecting device.
However, in the antenna device described in JP-A-2001-24431, the length of each antenna disposed upright on the grounded conductor is λ/4, and the interval between the central feeding antenna and each of the surrounding parasitic antennas is λ/4, so the antenna device at least requires a volume of a diameter of λ/2 and a height of λ/4. In JP-A-11-88246 and JP-A-2003-298331, the loop antennas each having a length of one wavelength are arranged on the sides perpendicular to each other, so the size of the antenna device increases. In the case of a system that detects a tire pressure of an automobile, a radio wave having a frequency of 315 MHz is used. In this case, the size of an antenna according to the above technique is about 50 cm, so it is difficult to use the antenna for the tire pressure detecting system. In the tire pressure detecting system, an antenna device that receives tire pressure data transmitted from four wheels is provided at a ceiling in the cabin of the automobile, so it is necessary to reduce the size of the antenna device as much as possible.